In a related art, there is an apparatus for fixing a toner image on a record medium in which a heating roller is heated by a halogen heater arranged at inside of the roller, a pressing roller is pressed to the heating roller by a coil spring or the like to form a nip portion between the rollers, and the record medium adhered with a toner is made to pass through the nip portion to be pressed and heated to fix. However, in order to achieve high speed formation, small-sized formation, high reliability, energy saving formation, and low cost formation by the type of bringing the rollers into contact with each other, the following problem is posed.
First, in accordance with high speed formation, it is necessary to prolong a distance of heating the toner on the record medium. A heating time period the same as that before increasing a speed is ensured. In order to prolong a length of the nip portion, it is necessary to make a surface of the roller elastically deformable and increase a load value, or increase a diameter of the roller. However, an increase in the diameter of the roller contradicts to small-sized formation. Next, there is conceivable a method of reducing an elastic coefficient of the surface of the roller and increasing the load value. In order to reduce the elastic coefficient of the surface of the roller, it is necessary to increase a thickness of a surface layer of the roller or reduce Young's modulus of the member. However, when the thickness of the surface layer of the roller is increased, a heat capacity of a total of the heating roller is increased, a heating time period necessary for elevating a surface temperature of the heating roller to a certain constant temperature is prolonged, as a result, a printing time period is prolonged. Although there is a method of maintaining the temperature of the heating roller by supplying heat at all times at standby, the method contradicts to energy saving formation. On the other hand, when the load value for bringing the rollers into press contact with each other is increased, it is conceivable that a pressure difference of the nip portion is brought about between a center portion and an end portion in an axial direction of the roller and a characteristic for fixing the toner becomes nonuniform. Further, there is a concern that a nonuniformity in a pressure distribution produces wrinkle of paper, as a result, reliability of the apparatus is deteriorated. When a bending amount is reduced, a rigidity of the roller is increased, or the roller is formed of a crown shape. However, when the rigidity of the roller is increased, the heat capacity of the roller is increased, and the heating time period is prolonged, which contradicts to energy saving formation. According to the roller having the crown shape, a number of roller forming steps is increased and low cost formation is difficult. Further, the pressure distribution of the axial direction depends on an accuracy of working the roller, as a result, there is a possibility of deteriorating reliability.
In order to resolve the problems, various proposals have been made (for example, Japanese Patent No. 3298354). According to the related art, there is a belt nip type of a fixing apparatus in which a pressing member for elastically deforming locally an outlet portion of a heating roller is brought into press contact with the rotatable heating roller a surface of which is elastically deformed to form a nip portion and a toner image on a record medium fed to therebetween is pressed and heated to fix. According to the type, by providing the pressing member to form a contact face from an inner side of an endless belt, a nip width can be formed to be long, sufficient heat can be applied to the toner and high speed formation of a printing speed can easily be dealt with. Further, the nip width can be formed to be long without enlarging a total of the apparatus to be able to deal with small-sized formation of a total of the apparatus. Further, the long nip width can be formed by a low pressure, thin-walled formation of the heating roller is facilitated, a heating time period up to a temperature proper for fixing can be shortened, printing can be started quickly and therefore, an energy saving effect can be promoted. Further, by pressing an elastic layer of the heating roller by a hard member, the elastic layer is locally warped at the nip outlet portion, a self stripping (self exfoliating) function is promoted and reliability thereof is high.
However, according to the belt nip type fixing apparatus, there poses a problem that an image shift is liable to be brought about owing to a speed difference by locally deforming the outlet portion. Further, by pressing an elastic layer of the heating roller by the hard member, the elastic layer is locally warped to produce a nip shape having a small curvature and therefore, a load (curl amount) applied to a sheet is increased.